1.(1+y/x)/(1-y/x) 化简 2.已知x/y=3,求（x的平方+2xy-3y的平方）/（x的平方-xy+y得

1。(1+y/x)/(1-y/x) =1-(y/x)^2=1-y^2/x^22（x的平方+2xy-3y的平方）/（x的平方-xy+y得平方）=(x+3y)(x-y)/(x^2-xy+y^2)=(x+3y)(x-y)(x+y)/(x+y)(x^2-xy+y^2)=(x+3y)(x-y)(x+y)/(x^3+y^3)=y^3(x/y+3)(x/y-1)(x/y+1)/y^3(x^3/y^3+1)=(3+3)(3-1)(3+1)/(3^3+1)=48/28=12/7